Concrete is a brittle material having low tensile strength and low strain capacity. To improve properties of concrete like tensile strength and strain capacity, fibre reinforced concrete and more particularly metallic fibre reinforced concrete has been developed.
It is known in the art that the properties of the fibres like fibre concentration, fibre geometry and fibre aspect ratio greatly influences the performance of the reinforced concrete.
With respect to fibre geometry it is known that fibres having a shape different from a straight shape provide better anchorage of the fibre in the concrete or mortar.
It is furthermore known that fibres not showing the tendency to form balls when added to or mixed with concrete or mortar are preferred.
Numerous examples of different fibre geometries are known in the art. There are for example fibres that are provided with undulations, either over the whole length or over part of their length. Examples of steel fibres undulated over their whole length are described in WO84/02732. Also fibres having hook-shaped ends are known in the art. Such fibres are for example described in U.S. Pat. No. 3,942,955.
Similarly, there are fibres of which the cross-section profile changes over the length, such as fibres provided with thickened and/or with flattened sections.
An example of a steel fibre provided with thickened sections is a steel fibre with thickenings in the form of a nail head at each of the extremities as described in U.S. Pat. No. 4,883,713.
Japanese patent 6-294017 describes the flattening of a steel fibre over its entire length. German Utility Model G9207598 describes the flattening of only the middle portion of a steel fibre with hook-shaped ends. U.S. Pat. No. 4,233,364 describes straight steel fibres provided with ends that are flattened and are provided with a flange in a plane essentially perpendicular to the flattened ends.
Steel fibres with flattened hook shaped ends are known from EP 851957 and EP 1282751.
Currently known prior art fibres for concrete reinforcement function very well in the known application fields like industrial flooring, sprayed concrete, pavement, . . . .
However, the disadvantage of currently known prior art fibres is the relatively low performance at ultimate limit state (ULS) when low or moderate dosages of fibres are used. For more demanding structural applications, like beams and elevated slabs high dosages, typically from 0.5 vol % (40 kg/m3) onwards and not exceptionally up to 1.5 vol % (120 kg/m3) are used to provide the necessary performance at ULS. These high dosages do not facilitate the mixing and placing of the steel fibre reinforced concrete.
Some prior art fibres do not perform at ULS as they break at crack mouth opening displacements (CMODs) lower than what is required for ULS. Other fibres, like fibres with hook shaped ends do not perform well at ULS as they are designed to be pulled out.